1. Field of the Invention
The invention relates to a power supplying device, more particularly to a power supplying device capable of providing adjustable output voltages.
2. Description of the Related Art
Since internal components of most electronic devices, such as televisions, monitors, stereo systems, computers, etc., require different direct-current (DC) voltages, a switching power supplying device capable of converting a DC input voltage into various DC output voltages is used in the electronic devices so as to supply the various DC output voltages respectively to the internal components of the electronic devices.
However, differences are usually present between the actual DC output voltages and the voltages required by the internal components due to an excessively large induced voltage at each output coil of an output transformer used in a conventional switching power supplying device. Normally, each output coil has an induced voltage that ranges from 1.5V to 6V depending on the circuit configuration and design parameters. Therefore, the conventional switching power supplying device requires an additional component for adjusting the output voltages thereof so as to comply with the voltages required by the internal components of the electronic device. For instance, a conventional LLC half-bridge power supplying device 1 as shown in FIG. 1 requires an additional DC to DC (DC-DC) converter 20. As another example, another conventional LLC half-bridge power supplying device 2 as shown in FIG. 2 requires an additional linear regulator 30.
For detailed circuitry of a conventional DC-DC converter, one may refer to the controller specification of model no. LTC1624 available from LINEAR TECHNOLOGY. An example of a conventional linear regulator is model no. LM117QML available from National Semiconductor.
However, the DC-DC converter not only involves complicated circuitry and is costly, but radiation problems such as radio frequency interference (RFI) are also easily generated therein. On the other hand, the linear regulator can only be used for reducing voltages, and power converting efficiency thereof is poor, especially when a difference between the input voltage and the output voltage is large. In addition, since voltage level of the input voltage exceeding the output voltage is consumed in the form of heat energy, this voltage level cannot be applied to an output circuit, and an external heat-dissipating mechanism is even required to accelerate heat dissipation.